Nitrogen, potassium and sulphur availability from residue-based fertilisers produced in Denmark applied singly or combined

In organic farming systems, nutrients taken up by crops should be replenished by the recycling of agricultural and urban organic waste to close nutrient cycles. However, the availability of organic wastes permitted for application in organic farming is limited, and organic farmers usually have to apply manure from conventional farms, which is considered to be contentious. Alternative nutrient sources acceptable for application in organic farming are therefore needed to supply available nutrients to maintain or increase organic food production. In this study, we investigated the ability of manure, digested manure, manure co-digested with the organic fraction of municipal solid waste, Fertigro (R) (a by-product from heparin production), and ash from straw to supply N, K and S to ryegrass grown in pots. We sought to establish whether the use of complex mixtures to obtain more suitable nutrient ratios can benefit plant growth. Digested manure and manure co-digested with the organic fraction of municipal solid waste gave on average 50% greater plant growth and 63% higher N recovery than manure, while no significant differences were observed in terms of K recovery. Fertigro (R) showed similar plant growth and nutrient uptake to the mineral S and N positive controls, while the addition of straw ash demon-strated similar K recovery as that observed for the mineral K positive control. The mixture of Fertigro (R) with manure co-digested with the organic fraction of municipal solid waste and ash was outstanding among the waste-based fertilisers in terms of amount of N, K and S supplied for plant uptake. However, further efforts are needed to confirm these benefits under field conditions.

Automated summary

In organic farming systems, the limited availability of organic waste for application requires the use of alternative nutrient sources to maintain or increase organic food production. This study investigates the ability of various waste-based fertilizers to supply N, K and S to ryegrass, showing that digested manure and manure co-digested with municipal solid waste are effective in promoting plant growth and nutrient uptake. Further research is needed to confirm these findings under field conditions.